Failure Analysis of Boiler Superheater Tube Elbow Cracking Due to Alkaline Stress Corrosion and Hydrogen Embrittlement

Literature Overview

This failure analysis paper, published in Materials Protection (Vol. 54, No. 10, 2021, pp. 154-157) by Chen Xingyang and colleagues from the Zhejiang Provincial Special Equipment Research Institute, investigates the cracking failure of superheater tube elbows in a 12Cr1MoVG steel boiler that occurred after only a few months of service. The research was supported by the Zhejiang Provincial Natural Science Foundation (Grant LQ20E010001). The findings reveal a combined degradation mechanism involving alkaline stress corrosion cracking (ASCC) and hydrogen embrittlement, which is a critical concern for power plant engineers operating high-temperature boiler systems.

Failure Mechanism Analysis

The authors employed a comprehensive non-destructive and destructive examination approach:

1. Optical microscopy: Revealed crack initiation sites and propagation paths on the elbow inner surface.
2. Microhardness testing: Identified localized hardness variations associated with the heat-affected zones and microstructural changes.
3. Scanning electron microscopy (SEM): Characterized the fracture morphology at high magnification, revealing intergranular cracking patterns.
4. Energy-dispersive spectroscopy (EDS): Identified the presence of alkaline species and hydrogen-related elements at the crack surfaces.

The failure mechanism was determined to be a synergistic interaction between two degradation processes:

• Alkaline stress corrosion cracking: Localized concentration of alkaline solution on the elbow inner wall, combined with high operating temperature and water flow, caused corrosion of the pipe interior surface. The alkaline environment attacked the grain boundaries of the 12Cr1MoVG steel, initiating intergranular cracks.
• Hydrogen embrittlement: The corrosion reaction generated trace amounts of hydrogen, which diffused into the metal matrix under the influence of residual stresses from the elbow forming process. Hydrogen atoms accumulated at grain boundaries, reducing the cohesive strength and triggering intergranular crack propagation.

Technical Parameters and Contributing Factors

Engineering Practice and Countermeasures

From a materials engineering perspective, 12Cr1MoVG is a widely used alloy for boiler tubes operating at elevated temperatures (typically 450-600°C). Its microstructure consists of tempered martensite with fine precipitates of M23C6 and MX-type carbides, which provide good creep strength. However, this alloy is susceptible to intergranular degradation under certain environmental conditions.

The residual stresses left from the elbow forming process (whether by hot bending, cold bending, or push-bending) are a critical factor. These stresses, particularly in the heat-affected zone where the microstructure has been altered by the bending temperature, create preferential paths for hydrogen accumulation and crack initiation.

Recommended countermeasures include:

• Implementing post-forming stress relief treatment (typically at 680-720°C for 12Cr1MoVG) to reduce residual stresses below the threshold for hydrogen embrittlement.
• Controlling the alkalinity of the boiler water chemistry to minimize localized alkaline concentration on the elbow inner surfaces.
• Conducting periodic in-service inspection of superheater elbows using eddy current testing or ultrasonic testing to detect early-stage intergranular cracking.
• Considering alternative materials with better resistance to alkaline stress corrosion, such as 9Cr-1Mo steel with improved microalloying.

Study Insights and Implications

This case study highlights the importance of considering the combined effects of environmental exposure, residual stress, and hydrogen generation in high-temperature boiler applications. The relatively short service life (a few months) suggests that the failure was driven by a potent combination of factors rather than gradual degradation over years. For engineers responsible for boiler maintenance and materials selection, this analysis underscores the need for comprehensive material characterization that includes susceptibility testing for both stress corrosion and hydrogen embrittlement, not just mechanical property verification.